The invention relates to a process for removing oxygen from hydrocarbon-containing gas mixtures.
Hydrocarbons such as ethylene, propylene or other hydrocarbons having from 1 to 7 carbon atoms are standard basic chemicals of the chemical industry. Gases of such hydrocarbons often contain impurities, for example oxygen, which can be a hindrance in the utilization of the hydrocarbon gases and in such cases should be at least partly removed from the hydrocarbon gases. A customary purification method is cryodistillation. However, this method is problematical for oxygen-containing hydrocarbon gases since such mixtures cannot be sufficiently separated by distillation and an accumulation of oxygen can instead occur when impurities are distilled off. In addition, cocondensation of hydrocarbon gases and oxygen can occur in cryodistillation, which is very problematical from a safety point of view. Accumulation of oxygen in hydrocarbon gases has to be avoided at all costs since oxygen is a hindrance for many uses of hydrocarbon gases and, particularly problematically, unstable or even explosive gas mixtures can be formed. In the membrane process too, sufficient separation does not occur, which can lead to accumulation of oxygen in the retentate or permeate and thus again to formation of explosive oxygen-containing gas mixtures.
The oxidative purification of gas mixtures, in which hydrocarbons and oxygen of the gas mixture are reacted with one another to form products which are not a hindrance or can easily be separated off, e.g. carbon dioxide and water, is also known. However, these reactions are strongly exothermic. The oxidative purification has found widespread use for gas mixtures which contain very small amounts of hydrocarbons and/or very small amounts of oxygen, for example industrial offgases or exhaust gases from automobiles. In the case of gas mixtures which contain relatively large proportions both of hydrocarbons and also of oxygen, heat is liberated in the oxidative purification in such considerable amounts that the formation of undesirable by-products or decomposition products or even an explosive oxidation reaction is to be expected.
In order to be able to carry out the oxidative purification at relatively low reaction temperatures, use has frequently been made of catalysts, for example ruthenium catalysts. However, even catalysts require certain minimum temperatures, the “light off” temperature, in order to obtain their catalytic activity, and these continue to be in a range which is problematical from a safety point of view in the case of gas mixtures having high contents of hydrocarbons and oxygen.
Thus, for example, U.S. Pat. No. 4,093,703 describes the oxidative removal of ethylene from gas mixtures containing up to 1.8% by volume of ethylene. US 2010/0048972 teaches the use of ruthenium catalysts for purifying ethylene from crackers. The oxygen content of the gases to be purified described therein is in the ppm range. US 2010/0048972 advises against the use of palladium catalysts for oxygen removal. EP 0499087 is concerned with the removal of traces of nitrogen oxide, carbon monoxide and hydrocarbons from oxygen-rich exhaust gases from gas turbines. The exhaust gases consist essentially of nitrogen. GB 883945 describes methods of purifying offgases from the oxidative reaction of ammonia to form nitric acid, in which nitrogen oxides and oxygen are removed by addition of unsaturated compounds and sulfides. These offgases, too, consist essentially of nitrogen. In Chemical Engineering and Processing 34, (1995), pages 469 to 478, van de Beld describes the total oxidation of ethene and propane over Pd catalysts supported on Al2O3. Rusu also discusses the oxidation of ethene in Environmental Engineering and Management Journal, 2003, Vol. 2, No. 4, pages 273 to 302. The most frequently used catalyst systems here are Pd or Pt on TiO2, Al2O3 or SiO2. In Catalysis Today, 122, (2007), pages 391 to 396, Hosseini recommends Pd and Au on TiO2 as catalyst for the total oxidation of propene. Catalysts for purifying exhaust gases from diesel engines are known from EP 2656904. The catalysts contain a catalytic coating comprising Pt, Pd and a carbon-storing compound such as zeolite and also a further coating comprising Pd and Au.
Owing to the fact that the handling and purification of hydro-carbon gases which are contaminated with considerable amounts of oxygen continue to present problems, such gas mixtures are frequently incinerated, which is fatal from an economic point of view since hydrocarbons such as ethylene are valuable petrochemical starting materials.